Tae In Kim

We propose a first-principles electronic structure model for describing a single vacancy defect. Vacancy defects in semiconductors and insulators form color centers, whose localized trap states enable quantum functionalities. The properties of these defect states are affected by their interactions with the surrounding lattice and spin environments. Extensive theoretical efforts have been devoted to understanding such defect-environment couplings within periodic supercell models. While these approaches have provided valuable insights, accurate description of charged defects or nuclear motion associated with electron-phonon coupling requires approaching the dilute-defect limit, which in turn demands prohibitively large supercells and computational cost. Our new model eliminates the artificial periodicity inherent to the supercell model. A single defect is introduced into the pristine crystal under the assumption that the electronic density far from the defect remains nearly identical to that of the pristine system. For the insulating LiH-1D system, this assumption holds well, as neutral and charged defects form localized color centers consistent with the model. In contrast, for the metallic H-1D system, delocalized electronic behavior in charged defects violates this locality condition, limiting the applicability of the approach. We expect that this model can be broadly applicable to investigations of charged color centers and their interactions with the surrounding environment in insulating and semiconducting materials such as diamond, h-BN, transition-metal oxides, and dichalcogenides.

Studies on quantum algorithms for ground-state energy estimation often assume perfect ground-state preparation; however, in reality the initial state will have imperfect overlap with the true ground state. Here, we address that problem in two ways: by faster preparation of matrix-product-state (MPS) approximations and by more efficient filtering of the prepared state to find the ground-state energy. We show how to achieve unitary synthesis with a Toffoli complexity about 7 × lower than that in prior work and use that to derive a more efficient MPS-preparation method. For filtering, we present two different approaches: sampling and binary search. For both, we use the theory of window functions to avoid large phase errors and minimize the complexity. We find that the binary-search approach provides better scaling with the overlap at the cost of a larger constant factor, such that it will be preferred for overlaps less than about 0.003 . Finally, we estimate the total resources to perform ground-state energy estimation of Fe - S cluster systems, including the Fe Mo cofactor by estimating the overlap of different MPS initial states with potential ground states of the Fe Mo cofactor using an extrapolation procedure. With a modest MPS bond dimension of 4000 , our procedure produces an estimate of approximately 0.9 overlap squared with a candidate ground state of the Fe Mo cofactor, producing a total resource estimate of 7.3 × 10 10 Toffoli gates; neglecting the search over candidates and assuming the accuracy of the extrapolation, this validates prior estimates that have used perfect ground-state overlap. This presents an example of a practical path to prepare states of high overlap in a challenging-to-compute chemical system.

We introduce the combination of the density functional tight binding (DFTB) approach, including onsite correction (OC) and long-range corrected (LC) functional and the state-interaction state-averaged spin-restricted ensemble-referenced Kohn–Sham (SI-SA-REKS or SSR) method with extended active space involving four electrons and four orbitals [LC-OC-DFTB/SSR(4,4)], to investigate exciton couplings in multichromophoric systems, such as organic crystals and molecular aggregates. We employ the LC-OC-DFTB/SSR(4,4) method to calculate the excitonic coupling in anthracene and tetracene. As a result, the LC-OC-DFTB/SSR(4,4) method provides a reliable description of the locally excited (LE) state in a single chromophore and the excitonic couplings between chromophores with reasonable accuracy compared to the experiment and the conventional SSR(4,4) method. In addition, the thermal fluctuation of excitonic couplings from dynamic nuclear motion in an anthracene crystal with LC-OC-DFTB/SSR(4,4) shows a similar fluctuation of excitonic coupling and spectral density with those of first-principle calculations. We conclude that LC-OC-DFTB/SSR(4,4) is capable of providing reasonable features related to LE states, such as Frenkel exciton with efficient computational cost.